drm/panel: simple: Add Powertip PH800480T013 drm_display_mode flags

Add missing drm_display_mode DRM_MODE_FLAG_NVSYNC | DRM_MODE_FLAG_NHSYNC
flags. Those are used by various bridges in the pipeline to correctly
configure its sync signals polarity.

Fixes: d69de69f ("drm/panel: simple: Add Powertip PH800480T013 panel")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230615201602.565948-1-marex@denx.de